thicker tubes will help with pinch flats but not very much, the higher the psi tire the less you have to worry about pinch flats the thicker/heavy duty tubes aka puncture resistant will help with thorns but not always, but they weigh alot more, the bigger the tire the bigger the tube because as the tube expands it gets thinner so if you have a 1.75 tube in a 2.25 then the tube is going to be thinner, but most tubes are universal like a 1.75 - 1.95 or 2.125 the smaller tire in back is so you can get plenty of clearance in the stays of the frame, and also some times for the chain, and the bigger tire in front for more traction when your carving taking sharper turns, ect, i would say that the biggest in back would be a 2.1 but you dont really need it, i run a 1.95 in back and a 2.2 in front